Tan^-1

Tanya860 Those are the built-in functions ATAN, ASIN, and ACOS, respectively.  (The "A" prefix refers to arc; arctangent is another way of saying inverse tangent, e.g.) 

You may want to watch this How To Evaluate INVERSE Trig Functions Using Excel video.

As I recall, some of the inverse functions result in ambiguous values over certain ranges; the Microsoft documentation that I linked to above do not mention that, so do some more searching, and verify what you find by testing them.